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Comparison of European Network Codes for AC- and HVDC-connected Sources

Nouri, Behnam; Arasteh, Amir; Göksu, Ömer; Sakamuri, Jayachandra Naidu; Sørensen, Poul Ejnar Published in: Proceedings of the 18th Wind Integration Workshop

Publication date: 2019

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Citation (APA): Nouri, B., Arasteh, A., Göksu, Ö., Sakamuri, J. N., & Sørensen, P. E. (2019). Comparison of European Network Codes for AC- and HVDC-connected Renewable Energy Sources. In Proceedings of the 18th Wind Integration Workshop

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Comparison of European Network Codes for AC- and HVDC-connected Renewable Energy Sources

Behnam Nouri, Amir Arasteh, Omer¨ Goksu,¨ Jayachandra N. Sakamuri, Poul E. Sørensen Department of Wind Energy Technical University of Roskilde 4000, Denmark Email: [email protected]

Abstract—Developing an integrated pan-European energy sys- of transmission system operators for electricity (ENTSO-E) to tem based on renewable energy sources (RES) has technical harmonize the network codes in . In this way, European and economic benefits. In this way, harmonized rules for grid network codes have been regulated for AC- and HVDC- connection of RES are required at the international level. Wind energy is one of the most promising renewable energy connected power plants in two exclusive codes [2-5]. worldwide. The integration of wind energy into the power system This paper presents the main aspects of the latest European is overgrowing through onshore and offshore installations. The network codes for the harmonization of European renewable European network codes have been drafted and regulated for energy generation and interconnections. In section II, the grid AC- and HVDC-connected power-generating modules (PGM) in interconnections of power plants in general are introduced. two separate international network codes. This paper presents the main aspects of the regulated European network codes and In section III, harmonization of European network codes is compares them. Accordingly, it is recommended to define the discussed. Finally, the main aspects of the European network European network codes based on RES connection type (AC codes are reviewed in section IV, and the main differences and HVDC) rather than the onshore and offshore categorization. and similarities between HVDC and AC connections in the Also, the main requirements for HVDC-connected generations network codes are illustrated. are being regulated all around Europe. Therefore, the integration of RES into European power systems via HVDC transmission would be easier. II.INTERCONNECTIONSOF RES Index Terms—European network codes, renewable energy RES can be connected to the main power grid through high sources, power-generating modules, HVDC-connected PGMs voltage direct current (HVDC) or AC transmission systems. The main applications of HVDC are the interconnection of I.INTRODUCTION non-synchronous networks, long-distance transport of electri- The integration of renewable energy sources (RES) into cal power, and submarine and underground cable transmission. the power system has been an important challenge for power Therefore, different types of power generation units can inter- system developers and operators. The connect through HVDC transmission systems from offshore has planned a fully renewable power grid by 2050, which or onshore areas. In this regard, HVDC transmission provides is called the pan-European to ensures a reliable, an economically viable solution for long distances, especially uniform and carbon-free European power grid [1-5]. In this in case of offshore power plants [5-8]. regard, a harmonized international rules for grid connection of Currently, wind generation is predominately onshore; how- power generations should be set out to provide a clear legal ever, there is an increasing interest in offshore wind generation framework for interconnection of different grids and facilitate the RES integration. Wind energy is one of the most promising renewable energy resources worldwide. The expected cumulative installed capac- ity of onshore and offshore plants (WPP) with an outlook to 2022 in Europe is shown in Fig. 1. Accordingly, the new installations of wind energy capacity at an average rate of 16.5 GW per year is expected in Europe. The total installed capacity would reach 253 GW with a share of 20% from offshore WPPs [1-3]. Harmonization of network codes can improve integration of this substantial amount of wind power along with other RES. The European commission has asked the European network

This project has received funding from the European Unions Horizon 2020 Fig. 1. Expected cumulative installed capacity of onshore and offshore WPPs research and innovation program under grant agreement No. 691714. until 2022 in Europe [3]. due to limited onshore sites, less public opposition to offshore and maturation of the associated technologies. Evaluation and comparison of different types of topologies and intercon- nections for WPPs have been done in the literature [8-12]. WTs are connected through medium voltage submarine cables typically at voltage level of up to 33-66 kV to the Offshore AC substation. The transformers in offshore AC substation step up the voltage to 132-200 kV for further power transmission with lower power loss. Offshore AC substation can be connected to the grid at shore either directly through AC cables or HVDC power transmission systems. Fig. 2 displays a typical structure of HVDC-connected offshore WPPs. As it can be seen in this figure, offshore power transmission consist of offshore and onshore HVDC converter stations, AC and DC cables, and offshore AC substations. Hence, the structure and behavior of the HVDC connection are different than a conventional AC Fig. 3. European distributed hubs concept for offshore grids in the North Sea grid connection which needs to be considered in regulations [4]. and standards for WPPs and, in general, RESs.

III.NETWORK CODES The pan-European super grid must facilitate the integration Power plants are required to provide a certain level of of RES, and manage the power systems interconnections reliability and stability. The integration of generated power caused by the pan-European electrical energy trade. In this into the power system is one of the most critical challenges way, the European network of transmission system operators in renewable energy technologies, especially wind energy. for electricity (ENTSO-E) is founded as an umbrella organiza- Motivated by the power system reliability and stability issues, tion for harmonization of European TSOs. Different scenarios grid interconnection requirements, called network codes, have for implementation of the pan-European super grid have been been developed by transmission system operators (TSOs) in offered by ENTSO-E and relevant industrial and academic different countries. In response to these codes, manufacturers partners [2-5]. As an example, Fig. 3 shows the European provide their products with features that cope with demanded distributed hubs concept for offshore grids in the North Sea, grid connection requirements. Also, the international collab- which is visualized in PROMOTioN’s Deliverable 12.1 [4]. orations in technology and market of RES demand uniform This concept presents a meshed international offshore power regulations and standards. grid using AC and HVDC transmissions with a high level of flexibility, reliability, and coordination among neighboring A. Harmonization of European Network Codes countries; however, it demands international regulations and The evolution of network codes has mostly happened at framework. national levels, but considering the recent trends towards an In this regard, the European Commission requested ENTSO- integrated pan-European energy system, focusing on a national E to harmonize the national network and market codes for level can lead to technical and economic problems in interna- Europe in consultation with all stakeholders. Consequently, tional level. The main aspects of network code requirements the European network codes are drafted by ENTSO-E with for the integration of renewable energy in European countries guidance from the agency for the cooperation of energy reg- at the national level have been presented in [13-16]. ulators (ACER). Thus, the network code on requirements for grid connection of generators (EU NC-631) [17], and network codes on requirements for grid connection of HVDC systems and DC-connected power park modules (EU NC-1447) [18] came into force as European Commission Regulation in 2016. The European network codes are serving as a framework for individual TSO network codes. Many TSOs currently are in an adaption process, to align with the European regulation. However, the current implementation of EU network codes is still done in national level, with some (less than before) differences between them. In EU NC-631, the European AC power systems have been divided into five synchronous areas as it is shown in Fig. 4 and listed below. • Continental Europe • Great Britain Fig. 2. Typical structure of HVDC-connected offshore WPPs [10]. • Nordic (East Denmark, , , and ) • (Ireland and ) or HVDC system is connected to a transmission or • Baltic (, , and ). distribution system, offshore network or HVDC system. In each of these synchronous areas, the power generating • Power park module (PPM): a unit or a group of units gen- modules are subdivided based on their PCC voltage level erating electricity. This unit is either non-synchronously and their generation capacity. The capacity thresholds need connected to the network or connected through power to be determined at national level to consider the national electronics which has a single connection point to a power system characteristics; however the maximum values transmission, distribution, or HVDC system. for lower thresholds are given in EU NC-631 as illustrated • Offshore PPM: a PPM located offshore with an offshore in Table I [17]. Based on this table, the offshore power connection point. generations can be considered as type D considering high • DC-connected PPM: a PPM connected to one or more power capacities (P > 75MW ) and high voltage connection HVDC systems via HVDC interface points. points (U ≥ 110kV ). • Power-generating module (PGM): either a synchronous PGM (SPGM) or a PPM.

B. Terminology of European Network Codes IV. EUROPEAN NETWORK CODESFOR RES Some of the terminologies used in EU regulations for The European network codes (EU NC) do provide a good network codes are shortly described in the following to have overview of the differences and similarities of network codes a better understanding of the regulations [17-18]: among European countries. In EU NC-631, the requirement for • Connection point: the interface at which the power- PGMs have been stated in four divisions: general requirements generating module, demand facility, distribution system for PGMs, requirements for SPGMs, requirements for PPMs, and requirements for AC-connected offshore PPMs. How- ever, many of these requirements are in common, especially between requirements for PPMs and AC-connected offshore PPMs. According to EU NC-631, the connection requirements for offshore AC power generation units are similar to onshore power generations, except for following two conditions [17]: First, the relevant system operator (SO) modifies the require- ments. Second, the connection of power plants is via an HVDC connection or an asynchronous network. Therefore, the network codes for AC-connected onshore and offshore PGMs are mostly similar. In other words, the main differences in the European network codes are between AC- and HVDC-connected PGMs rather than onshore or offshore. This section reviews and compares the EU network codes for AC- and DC-connected PGMs. It should be noticed that the European network codes propose regulations at the international level, and the national implementations of them will not be discussed in this paper. A. Frequency Stability Requirements Frequency stability refers to the ability of a power system Fig. 4. Different synchronous areas (regional groups) in Europe according to to maintain the frequency within a specified acceptable range ENTSO-E [5]. during a severe system upset [19]. PGMs should be capable of remaining connected to the network based on frequency TABLE I stability requirements, PGMs need to remain connected to MAXIMUM VALUES FOR LOWER THRESHOLD OF CAPACITY, WHICHPOWER PLANTSCANBEADDRESSEDTYPE A,B,C OR D [17]. the network for a specified time, which depends on value of frequency deviations. The minimum operating time in different synchronous AC Synchronous U < 110kV U ≥ 110kV regions as well as HVDC-connected PPMs as a function of area A B C D frequency deviation are depicted in Fig. 5. The AC-connected Continental 0.8 KW 1 MW 50 MW 75 MW Europe power plants consist of the five synchronous areas, which are Great Britain 0.8 KW 1 MW 50 MW 75 MW listed in section III-A. According to the European network Nordic 0.8 KW 1.5 MW 10 MW 30 MW codes, the time duration for Nordic, Baltic and Continental Ireland 0.8 KW 0.1 MW 5 MW 10 MW European countries are identical. Also, the values for GB Baltic 0.8 KW 0.5 MW 10 MW 15 MW are similar to DC-connected PPMs. Apart from the required operating time, there are two main differences in frequency 1.05 pu for higher voltages. According to Fig. 6, the voltage- time duration requirements for DC-connected PPMs are more demanding. In addition, the requirement for type D PGMs in Baltic and Continental countries are more demanding than other regions, which means they should be more robust against voltage deviations. Another essential option for voltage support is reactive power (Q) capability to control the voltage locally across a power grid [13]. The requirements for reactive power ca- pability at active powers P ≤ P max are specified by P- Q/Pmax and U-Q/Pmax profiles in the network codes for type c and D PGMs, and AC-connected offshore PPMs. The ratio Q/Pmax is defined as ratio of the reactive power (Q) and the maximum capacity (Pmax). The maximum ranges of reactive power requirements for PGMs are same as AC- Fig. 5. The minimum operating time in different synchronous AC regions as connected offshore PPMs and the only difference is that the well as DC-connected PPMs as a function of frequency deviation [17-18]. maximum range of Q/Pmax in GB is 0 or 0.33 pu (based on the configuration of PPM) offshore, while onshore it is 0.66 stability requirements of AC- and HVDC-connected power pu [17]. Besides, concerning the injection of fast fault current plants as follows. in asymmetrical (1-phase or 2-phase) fault conditions, there is no specific requirement in EU codes, but the relevant system • One difference is in the rates of change of frequency operator can specify a requirement for asymmetrical current (RoCoF) of the system. In DC-connected PPMs, the value of RoCof is specified in the associated network code (±2 Hz/s), while in AC-connected power plants individual TSOs can specify RoCoF. • The second difference is that the DC-connected power plants, which connect with more than one control area, should be capable of delivering coordinated frequency control [18], which is not a requirement for AC-connected power plants. Regarding the RoCoF in DC-connected PPMs, the HVDC converters control the frequency of the local AC grid. Conse- quently, frequency variation is smooth and RoCoF is expected to be way below the specified limit of 2 Hz/s.

B. Voltage Stability and Reactive Power Requirements Power systems should be able to maintain steady voltages (a) 110kV ≤ U < 300kV at all buses in a specified range after being subjected to a disturbance. Voltage instability occurs in the form of a progressive fall or rise of voltages of some buses which may lead to tripping of transmission lines and cascading outages [19]. The minimum time duration, during which a PGM must be capable of operating over different voltage ranges without disconnecting, is given in Fig. 6-a and Fig. 6-b. In case of DC-connected PPMs, the periods for a voltage range of 1.118 to 1.15 pu in a voltage range of 110kV ≤ U < 300kV and 1.05 to 1.15 pu in range of 300kV ≤ U ≤ 400kV should be specified by the relevant system operator. In EU NC-631, the voltage-time duration requirement is only given for type D PGMs and AC-connected offshore PPMs and the specified values for them are very similar. The only difference is that the upper voltage level of unlimited operation range for (b) 300kV ≤ U ≤ 400kV Ireland is always 1.1 pu in case of offshore PPMs, while for Fig. 6. The minimum required operating time as a function of voltage- type D PGMs, it is 1.118 pu for voltages below 300 kV and amplitude for AC- and DC-connected PPMs [17-18]. generation consisting A, B, C and D according to Table I. The given parameters by EU NCs are only for low voltage ride- through (LVRT) capability and there is no FRT capability for type A PGMs. Fig. 8 illustrates the most possible demanding LVRT capability profiles for SPGMs, PPMs and HVDC station converters. Accordingly, the LVRT capability of HVDC station is enormously demanding. Also, it is expected that PPMs (type B, C and D) should tolerate deep faults better than SPGMs. In addition, the network codes for type D generation (both D SPGM and D PPM) are more demanding rather than type B and C; especially, since they should tolerate 100% deep faults at least for 140 ms period. HVDC-connected PPMs may not detect the occurrence of (a) FRT profile for PGMs faults in onshore AC grid, but it can affect the level of active power that can be transferred to the onshore side and therefore subject the offshore PPMs to a load rejection [20]. Also, the individual TSOs should specify the requirement for FRT capabilities in case of asymmetrical faults.

D. System Restoration Requirements System restoration requirements aim to the capability of reconnecting of PGMs to the network after an incidental disconnection caused by a network disturbance. The system restoration requirements for AC-connected onshore and off- shore PGMs consist of black start, island operation, and quick re-synchronization capabilities. In an HVDC system with black start capability, after system (b) FRT profile for HVDC station shut down, one converter station should be energized by a storage system. Then, the HVDC system should be able to Fig. 7. The voltage-period profile for FRT limits of the voltage at the energize the busbar of the AC substation to which another connection point [17-18]. converter station is connected. The relevant TSO determine the time frame of the black start operation for HVDC systems injection. [18]. However, the interactions between the AC and DC sides of HVDC connections have not been considered in the grid C. Robustness Requirements codes yet [21-22]. According to [22], the interactions between The aim of robustness requirements is that in the event AC and DC grids can be defined such as AC transmission of power oscillations, PGMs be able to retain steady-state reserve, DC transmission reserve, AC power flow control, and stability when operating at any operating point of the P-Q DC power flow control. capability diagram. Fault ride-through (FRT) capability is the main requirement for robustness, which refers to the ability of a PGM to remain connected to the power system during short periods of under-voltage or over-voltage. Fig. 7-a shows the voltage-time profile in network codes representing lower limits of the line voltages at the connection point during a symmetrical fault for PGMs. In addition, FRT profile of an HVDC converter station is shown in Fig. 7-b. The HVDC systems should be capable of finding stable operation points during and after any change in the HVDC system or the connected AC network. Table II shows the specified range of parameters for FRT profiles in European network codes. The FRT profile parame- ters depend on PGMs and connection types. As it is introduced in section III, PGMs are either synchronous PGM (SPGM) or non-synchronously connected (PPM). Also, the PGMs are Fig. 8. The most demanding LVRT capability profiles for different PGMs distinguished by power rating which addresses the type of and HVDC station [17-18]. TABLE II PARAMETERSFORTHE FRT CAPABILITY OF AC-CONNECTED PPMSAND HVDC CONVERTER STATION [17-18].

Voltage parameters [pu] Time parameters [s] SPGM PPM HVDC SPGM PPM HVDC Uret *D:0, **BC: 0.05∼ D:0, BC:0.05∼ 0∼ 0.30 tclear 0.14∼ 0.14∼ 0.14∼ 0.25 0.3 0.15 0.15or0.25 0.15or0.25

Uclear D:0.25, D:Uret,BC : - trec1 tclear(D : tclear 1.5∼ 2.5 BC:0.7∼ 0.9 Uret ∼ 0.15 tclear ∼ 0.45)

Urec1 D:0.5∼ 0.7,BC : Uclear 0.25∼ 0.85 trec2 trec1 ∼ 0.7 trec1 trec1 ∼ 10 Uclear Urec2 D: 0.85∼ 0.9,BC : 0.85 0.85∼ 0.90 trec3 trec2 ∼ 1.5 1.5∼ 3 - 0.85 ∼ 0.9and ≥ Uclear *D: type D and, **BC: type B and C power plants.

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